Chevelle Frame Rail Narrowing

[HEEP]

Ok, I have searched and read every thread on this subject. However, I could not see where anyone stated just how much was gained. What I am looking for is just how much you can gain from reducing the width of the frame rail itself. This is not narrowing the rails to create a four link drag race type setup. I am looking to add a larger tire in the rear. I'm set on a minimum of a 19" wheel, and my understanding is that I can get a 10" width under the well without any modification.
I have even thought of going full force and ordering these and grafting them in.

http://autoweldchassis.com/spec.ivnu

these are 3" tube that are supposed to fit under a stock body according to the company.

I am at the place where i need to make this decision so I can either move to the next step or stat cutting the frame.
Thanks in advance,
HEEP